76 NOTES AND MERIORANDA. 



chromatic aberration without reference to the spherical aberration, and 

 correcting the latter by one or more negative crown-glass lenses of the 

 proper focal relations to the others. The accompanying drawing 

 represents a cross section of the object-glass. A is the double 

 concave flint-glass lens ; B and C crown-glass lenses, of appropriate 

 focal projiortions ; and D an additional ne- 

 gative crown-glass lens, its purpose being 

 the completest correction possible of the 

 spherical aberration. The double concave 

 form is the best for the highest possible 

 correction of the chromatic aberration, the 

 ordinary concavo-convex form having been 

 adopted only as a compromise aiming at the correction of both aber- 

 rations at the same time, which can only be imperfectly attained 

 under such circumstances. By this means the objective can be made 

 almost absolutely achromatic, leaving the sjiherical aberration to be 

 corrected by other and independent means, viz. by tlie sj)ecial nega- 

 tive crown-glass lens, D (or, if preferred, more than one), concavo- 

 convex in shape, the concave surface of which has a shorter radius 

 than the convex surface of the positive crown-glass lens nest to it, so 

 that between the two a space remains in the shape of a meniscus. 



The loss of illuminating power on account of the increased 

 number of surfaces can be reduced to a minimum by cementing the 

 adjoining surfaces ; the loss being further reduced in comparison 

 with a double object-glass by having the outer surfaces consisting of 

 crown glass, the loss of light on such surfaces being less than on 

 flint-glass surfaces. It will, moreover, be preponderatingly com- 

 pensated by the better correction of the aberrations and the greater 

 clearness and sharpness of definition resulting therefrom. 



British Acari— Oribatidse. — Mr. A. D. Michael has sent a paper 

 to the Society (which cannot be published yet for want of space) 

 giving the results of his researches among British Acari of the family 

 Oribatidce, conducted during the past year in conjimction with Mr. C. 

 F. George. Forty-four species have been found, of which only three 

 or four have been previously recorded as British. Of these forty-four 

 species, three are believed to be entirely new, viz. two species of the 

 genus Tegeocranus which Mr. Michael proposes to call respectively 

 T. lahyrinthicus and T. eloncjatus, and one which he proposes to make 

 the type of a new genus to be called Scutovertex, the species being 

 called sculptus. The new species are fully described and figured. 

 The life-history of Tegeocranus latus, Noilirus tJieleproctus, &c., of 

 which the larvae and nymphs were not previously known, have been 

 traced, and are described and figured. 



The Structure of the Nerves in the Invertebrata. — The histolo- 

 gical characters of the nerves, whilst determined with precision for the 

 Vertebrata, are imperfectly known in the other divisions of the animal 

 kingdom. Their exact determination is nevertheless important from 

 all points of view, for the examination of the external form alone is 

 insufficient when we wish to know if such or such part among inferior 



